Various types of systems have been devised in the past for use in detecting the presence of an object within an area or space under surveillance. Many of these systems find particular application in vehicles for the purpose of alerting the driver of the presence of adjacent objects and cars. The systems employed in vehicle applications are sometimes referred to as collision avoidance systems.
Numerous types of differing technologies have been employed in previous collision avoidance systems. For example, one known type of system is based on ultrasonics. A high frequency sound wave is emitted into the area under surveillance and reflected sound waves having the frequency of the emitted sound are analyzed to determine whether an object is present in the monitored area. Ultrasonic-type systems are rather slow in terms of response time and are particularly subject to various kinds of interference due to wind buffeting and eddies as well as other sources of interfering noise which reduces the reliability of the systems. Moreover, the ultrasonic transducer employed in these systems must be mounted in the open air, thus subjecting it to airborne dirt and moisture which can eventually interfere with the proper operation of the system.
Another type of known collision avoidance system uses radio frequency transmitters and receivers or radars. These latter-mentioned systems, while relatively effective in terms of performance, are relatively expensive and therefore have not gained widespread use in common passenger vehicles.
Optical systems are also known which measure the time of flight for light transmitted from the vehicle, to the object and back. However, these systems also require expensive components and relatively complex circuitry.
Accordingly, there is a need in the art for a low cost, relatively simple device for detecting the presence of an object within an area under surveillance which may be employed, for example, in vehicles to avoid collisions.